1. Field of the Invention
The present invention relates to a method for separating vinyl chloride (hereinafter referred to simply as VCM) via cooling and a purification step such as distillation, from a gas formed by thermal cracking of 1,2-dichloroethane (hereinafter referred to simply as EDC). Particularly, the present invention relates to a method wherein the gas formed by thermal cracking of EDC is cooled in specific steps, and formed coke is efficiently separated to prevent a trouble in the subsequent step for separating VCM.
2. Discussion of Background
Heretofore, as a method for producing VCM, a method is known wherein EDC is thermally cracked at a temperature of from 450 to 550.degree. C. in a thermal cracking furnace, and the formed cracked gas is cooled and then VCM is separated by distillation. The high temperature cracked gas discharged from the cracking furnace contains VCM and hydrogen chloride as cracked products, and noncracked EDC mainly, and it is supplied to a quenching tower as it is, or it is indirectly cooled by a heat exchanger and then supplied to a quenching tower, where the heat of the high temperature cracked gas is recovered.
In such a method, it is known that coke will deposit in the cooling pipe of the heat exchanger used for cooling the cracked gas to cause deterioration of the performance, increase of the pressure loss and clogging of the pipe, and it is impossible to conduct a continuous operation over a long period of time.
Therefore, JP-B-6-92328 proposes to carry out heat exchange of EDC to be supplied to the thermal cracking furnace with a high temperature cracked gas discharged from the cracking furnace by permitting the cracked effluent gas to flow through a pipe at a flow rate of at least 5 m/sec and less than 20 m/sec and cooling the cracked effluent gas to a temperature of from 180 to 350.degree. C., whereby it is said to be possible to prolong the continuous operation period of the thermal cracking furnace. However, in this method, the cracked gas cooled in a heat exchanger is introduced as it is to the conventional quenching tower, then quenched to e.g. 80.degree. C. and then sent to the subsequent step, whereby the bottom liquid is withdrawn in a large amount, and it is difficult to sufficiently separate coke from the bottom liquid before supplying the bottom liquid to a hydrogen chloride tower. Further, JP-A-6-219977 discloses a method in which a specific cooler which is single pipe of from 150 to 250 mm and which employs the bottom liquid of an EDC distillation tower as a cooling medium, is used for indirectly cooling the cracked gas, and the cracked gas is permitted to flow in the single pipe of the cooler under a pressure of from 0.9 to 1.4 MPa and cooled at an average cooling rate of from 15 to 45.degree. C./sec until the cracked gas temperature at the outlet of the cooler becomes from 250 to 350.degree. C., and EDC thermally recovered by a cooling medium in the cooler is directly refluxed to the EDC distillation tower, whereby the cracked gas withdrawn from the cooler is quenched to a level of from 40 to 150.degree. C. in a quenching tower, and VCM is recovered in the subsequent step, but the bottom effluent withdrawn from the bottom of the quenching tower is separately treated without passing through a distillation purification step.
This method has a merit such that the continuous operation periods of the thermal cracking furnace and the cooler can be prolonged equally to sixth months. However, the effluent from the bottom of the quenching tower is separately treated, thus leading to a disadvantage such as a loss of EDC or VCM in the effluent.